Teacher resources and professional development across the curriculum
Teacher professional development and classroom resources across the curriculum
Interviewer: What makes Mars different from Earth?
ANDY: If you look at a picture of Mars from one of the NASA spacecraft, the big difference is obvious right away and that is whereas Earth is this blue planet whose distinguishing feature is water, Mars is this red planet whose distinguishing feature is rock. There's very little water present on Mars. There's certainly some ice. There's a tiny, tiny bit of water vapor but basically on present day Mars, liquid water is actually not stable so it's a dry, cold planet.
Interviewer: Going back to the formation of the solar system, were Mars and Earth cut from the same cloth and a divergence took place or was it just the conditions on Mars that made it so different?
ANDY: All of the planets that orbit around the sun were formed in the same set of formational circumstances. That is there was an event of solar system formation, but each of the planets has certain peculiar features that are a product of the place within the solar system where that planet accreted. So Earth and Mars have a lot of things in common. They're both rocky as opposed to Jupiter and Saturn which are mostly made of gases. On the other hand, because it was farther away from the Sun, because of just the dynamics of how the planets first accreted, Mars probably had a lower inventory of water and other comparable chemicals than Earth did from the get go so the present day dearth of water on Mars probably goes all the way back to its earliest history that it was just less well endowed.
Interviewer: Is there evidence that there was liquid water on Mars?
ANDY: It turns out if you ask about the kinds of materials from which Mars formed - and we know something about those materials because we have evidence of them in certain types of meteorites - they do contain water. It's not that Mars was a bone dry planet from day one. If you just calculate the size of Mars and the amount of water in the materials that contributed to the formation of Mars, that's at least enough water to cover the planet in something like fifty meters of water and there are some people who would argue there were substantially larger bodies of water on Mars in the past. There certainly is evidence from the surface morphology of Mars for channels that were cut by water. We've learned a lot about Mars from the recent landers, the rovers that have been active on Mars. The chemistry of the rocks they see requires liquid water to form. So it's not that water was absent on Mars; it's at least possible that for the first 500 million years or so of Martian history there was enough water that life could have arisen, but however similar Earth and Mars were in their earliest history, they started going down very different environmental pathways four billion years ago, and really by that early time in Martian history, the window for life was really closing.
Interviewer: What was happening at that point in time on Earth?
ANDY: One of the things that we know is that on Earth there was a lot of water and carbon dioxide and nitrogen gas and a number of other gases that were brought together during accretion and that accumulated in the atmosphere, and we know that the Earth cooled sufficiently so that fairly early in its history, within the first few hundred million years, it must have been cool enough to have liquid water, and through time, that liquid water accumulated. It basically left its sort of vapor-rich atmospheric phase and accumulated in ocean basins so that over the first billion years of Earth history we came to have a recognizable planet in the sense of one in which there were oceans and there were some type of continental rocks sticking out from those oceans. And there was an active tectonics that caused this material to move around and accrete larger continents, and sometime within that window, life arose, presumably as a product of those very kinds of geological and chemical reactions that shake the environment.
Interviewer: In previous interviews you've said that life is a planetary phenomenon and that if you want to look at where life began, you have to look at the conditions on the planet at that time.
ANDY: I think that there's no question that life really is a planetary phenomenon, that life arises on a planet as the product of planetary processes that are in play when the planet is young. Life is sustained, on this planet at least, by platectonics and other geological processes that continually renew the supply of materials needed by organisms. What's most fascinating to me is that in time, life can emerge as a set of planetary processes that are important in their own right. For example, if we look on this planet, our atmosphere contains about twenty percent oxygen and the only plausible source for that oxygen is photosynthesis by plants, algae, and cyanobacteria so life is not only responding to the physical conditions of a planet. There's a feedback so that life becomes an important determinant of climate of ocean and atmospheric composition and even probably of the shape of the land and sea.
Interviewer: How do you know about things that happened billions of years ago?
ANDY: The way we know really goes back to the most basic, and to my mind, most wonderful fact about this planet which is that we live on a planet that records its own history in sedimentary rocks that are laid down one layer on another through time so that if you go to the Grand Canyon, you'll see a magnificent history that's hundreds of millions of years long laid down in those wonderful red, white, and gray beds that accumulated through time. One of the ways we know about the history of life is that many of these rocks contained fossils. Right now we're sitting in the fossil collections of the Museum of Comparative Zoology at Harvard. There are literally millions of fossil specimens in these collections. They were painstakingly collected by paleontologists over the past hundred years or more. It turns out that each group of fossils is associated with a particular time in Earth history with particular environments as reconstructed from the rocks. When you step back from the individual pages of that history that we see in a single bed and look at the volume as a whole, which is all of the sedimentary rocks exposed all over the earth, there is a consistent and coherent pattern of succession of life over nearly the entirety of our planet's history.
Most of us have been to a museum. There's a very good one here at Harvard where you can see the kinds of fossils that most of us learn about from daily life. There are fossils of dinosaurs; there are fossils of trilobites and other marine creatures and they actually provide a long history of life. Dinosaurs take us back about two hundred million years into our planet's past. Trilobites take us back more than five hundred million years into our planet's history, but we know from the radioactive decay of minerals that our planet is about four and a half billion years old so we have to ask what kind of life was there on this planet before trilobites and sponges and corals the other kinds of sea creatures that become apparent at the beginning of the so-called Cambrian period, about five hundred and forty million years ago? Darwin was very concerned with this issue. In fact, he didn't know, at the time he wrote The Origin of Species, of any evidence for life older than trilobites and things of comparable age, and he actually stated in The Origin of Species that that was a problem and that he envisioned that the solution would be that there was a much deeper, longer history of life that remained undiscovered at his time.
Now Darwin actually had in mind a long, long history of animal life that gradually showed divergence over many millions of years to lead up to the trilobites, but I can show you this slab, which is a cast of some of the earliest animals we know,in which there are some little, flat tubular-like organisms that come from Australia. They are so-called Ediacaran organisms and this is fairly close to the earliest evidence of animal life we have. It's very different from anything living today. It is not an obvious progenitor of the trilobites that we see later on, but it is early evidence of animals. How old is this stuff? Well it's only about ten million years older than the beginning of the Cambrian Period. The rocks in which these fossils were found are probably about 550 million years old and the oldest microscopic evidence for animals only goes back a little more than 600 million years so, in a sense, Darwin was right. There is an older record of animals than the trilobites of the Cambrian but not that much. We still have three billion years of earth history that we can ask biological questions about prior to these oldest animals.
Interviewer: Tell me about the two fossils you're holding.
ANDY: What we're looking at here is a cast of two fossils that really rank among the earliest evidence of animal life that we have. Both of them are sort of flat, disc-like objects. If you look closely, you'll see that each one of them is made of a series of tubes that are aligned together as a sort of central organizer. It's reasonably likely that these are colonies of simple animals, a little bit like a modern Portuguese Man-of-War rather than one large wormlike organism. There's a lot of argument about what these kinds of organisms really were. That is what are their closest relatives among living animals? How did they function? The important thing is that many of the earliest animals we see have this quilt-like construction. None of the earliest animals we see are very similar to the trilobites and other recognizably modern architectures that we see in the Cambrian Period. It is simply an early extinct and in some ways mystifying first record of animal life on this planet.
Interviewer: And this is the earliest animal life you can find?
ANDY: These are the earliest visible evidence of animal life, at least visible to the naked eye. Fossils like this go back to 575 million years. We do have a slightly older record of microscopic evidence of animals which rather remarkably consists of eggs, egg cases, and embryos of tiny early animals and those we now know go back at least to 632 million years. They're actually very accurately dated by volcanic ash falls. So even the simplest, smallest records of animal life that we have go back only a little more than six hundred million years while the sedimentary record in which we could look for life goes back an additional three billion years.
Interviewer: If you have a record of life going back about 600 million years but the sedimentary record goes back three billion years, what happened during that time?
ANDY: Animal life is our own branch of the tree of life. It's one that we're particularly interested in. It's one that leaves an indelible and interpretable imprint on the rocks in terms of fossils and yet we're finding that animals characterize only the last fifteen percent or so of the recorded history of our planet. So what came before that? Is there evidence of some other kind of life that has a deeper historical record than animals. We know that there's other kinds of life. If you take all the cells within your body, it turns out there's more bacterial cells inside your body than there are cells of you. So we live in a world that is in many important ways shaped by microorganisms - bacteria of various types and another group of micro organisms called the Arkeia that was only really discovered to be a distinct group about thirty years ago and many cells that have nuclei but are only single celled organisms -algae, protozoans - so, in a sense, in terms of the diversity of life on this planet, animals are the tip of the iceberg and when we look at the genealogical relationships among all of these types of organisms, animals are late branches on the tree of life. What we should be looking for is not an ever deeper record of animals but a record of that greater and older diversity of life which is the record of microorganisms.
That gives rise to another question which is why should we expect that tiny, fragile organisms like bacteria should actually leave a signature in the geologic record, and the good news is that they leave quite a signature. They leave a signature in several different ways. It turns out that under certain circumstances of deposition, we can actually preserve as cellular remains a record of algae, protozoans and bacteria. For example, this rock that we are looking at, which was deposited in what's now Spitsburgen, the arctic tip of Europe, this rock formed between seven and eight hundred million years ago and if you look at the black chips in this rock, they are made of a substance called silica, SIO2. It's the material of window glass. Silica acts almost like a plastic. When it's deposited, it will actually incorporate at a tiny, tiny scale everything that it surrounds and if we would take a paper thin slice of this rock and look at it under the microscope, we would actually find the cellular remains of algae, of protozoans, of different types of bacteria. It turns out that we can take this principle of looking in a microscopic way at very old rocks and find that the fossil record of bacteria- like organisms goes back more than three billion years so it confirms this idea that life is very old and animals are relatively latecomers to the evolutionary scene.
With a rock like this, we can establish in principle that microorganisms leave a signature in sedimentary rocks. They leave it through fossils; they leave it through particular kinds of features that accumulate when microorganisms interact with the physical environment. They leave it in terms of actual preserved molecules that can be identified as the products of different kinds of organisms and they actually leave a record in the chemical composition of the limestone itself that reflects the activity of photosynthesis. So this rock, even though it has no bones or shells or anything like that, is just dripping with evidence of life. The question is how far back can we go in our planet's history and find at least a broadly similar record. This rock was deposited about three and a half billion years ago in what's now South Africa and interestingly it also carries some of the physical and chemical signatures of biology. In essence we can look through the entirety of the preserved sedimentary record that has been chronicling earth history and everything we see in that record seems to reflect the early emergence of life.
Interviewer: Can you describe some of the features of the rock that you're holding?
ANDY: What's interesting about this rock is that it's made of silica again, SIO2 or quartz. Silica has no intrinsic color so the reason this is black is because it preserves organic matter that was deposited in sediments three and a half billion years ago. That organic matter has chemical features that we generally associate with life, as well as some wavy and bulbous laminations, if you will, and those are features called stromatolites; many stromatolites are the products of microbial mat activity. Interviewer: How do you know those stromatolites are evidence of life? ANDY: I think if you only had this rock to work with, you might be hesitant to even discuss whether it was evidence of life but we can go to a number of places today, places like the Bahamas, Western Australia, other places where microbial mats are forming stromatolitic structures and we can find a pretty much unbroken record of these kind of structures in limestones that connect this very ancient rock with the present day where we can actually observe the processes that lead to this kind of pattern.
Interviewer: What role does oxygen play?
ANDY: This rock with its stromatolites tells us that life was present early in our planet's history, but let me show you another rock also three and a half billion years old from Southern Africa, and this rock looks very different. You can see here it's bright red and light green, and it turns out the bright red is iron oxides, a mineral called hematite. The bright green is iron carbonate, a mineral siderite. What we're looking at is a rock called iron formation that contains a great deal of iron in it and what's interesting about this kind of rock is that iron formation formed all over the world for the first half of earth history but essentially has not formed since that time. In principle this kind of rock could not form in today's oceans because you can only transport iron through oceans when there is no oxygen present so in order for this kind of rock to form, iron had to enter the oceans either through erosion of continents or through hydrothermal processes at mid ocean ridges. It was then freely transported through the oceans until it came in contact with something that caused it to become oxidized and get precipitated as these minerals. It could have bumped into a little bit of oxygen in the surface of the ocean, but very likely what caused this iron to change its chemical state and drop out of solution was photosynthetic bacteria so rocks like this tell us that for the first half of our planet's history there was very little, if any, free oxygen gas in the atmosphere and oceans. You and I wouldn't have lasted five minutes on the early earth, but that changed.
It started to change about 2.4 billion years ago when we start seeing chemical evidence of minerals that form only when oxygen is present, and the agency for that, the source of the oxygen, has to be photosynthetic bacteria. That is the cyanobacteria which take carbon dioxide and water and in the presence of light make that into sugars and oxygen. That's the source of this oxygen. Now what's interesting is that all the chemistry of the rock record tells us that we went from a planet essentially without oxygen to a planet with some oxygen at that midpoint in our planet's history, but even after that event was done, if you had been around two billion years ago or one billion years ago, you still would have only lasted five minutes because the amount of oxygen that accumulated in the atmosphere and surface ocean was fairly small, probably maybe at best about 10 percent of the amount of oxygen in our current atmosphere. Recently evidence has accumulated that we didn't have enough oxygen to support the biology of large animals, to support our own biology, until about 580 million years ago so interestingly enough, just when the chemistry of rocks tells us that oxygen has finally accumulated to high levels, the fossil record tells us that animals are evolving in that new set of environments. There's this close waltz, if you will, between earth's physical environment, the history of earth's physical environments, and the history of life.
Interviewer: Are we totally dependent on signs of bacteria?
ANDY: Well, in a sense, we are dependent on cyanobacteria. It turns out that the cyanobacteria are all around us; we just don't recognize them. We look outside and we see plants which are providing about half of the photosynthesis on this planet. If you took a plant course in high school, you probably learned that the place in a plant where photosynthesis is carried out is the chloroplast. Well, where do chloroplasts come from? Remarkably, chloroplasts originated as cyanobacteria that were captured by a protozoan more than a billion years ago and eventually reduced to metabolic slaves, so, in a sense, photosynthesis that uses water and produces oxygen was only ever invented once. It was evolved by the cyanobacteria and then those cyanobacteria came to be functional parts of algae and plants by essentially being swallowed and becoming obligate elements of another cell.
Interviewer: So how do you talk about photosynthesis as an overall reaction that's very complex?
ANDY: The question is really how do we draw inferences about the nature of photosynthesis and other metabolisms that microorganisms had in earth's deep past and how do we draw inferences about the nature of those environments, and the one word answer is chemistry. Here's a rock that was deposited about a billion and a half years ago in Northern Australia. The particular specimen that we're looking at here is nicely rounded because we actually drilled into the Australian countryside and pulled out rock that had been sitting, in this case, several hundred meters below the surface of present day Australia. We do that because it allows us to have very, very fresh rock that has not been contaminated with modern chemicals and has not been subjected to weathering processes that would go on at the surface.
This turns out to be a rather remarkable repository of information on biological and environmental history. If you look at it, it's black. It's black because there's a lot of organic matter in it and we can do a couple of things to that organic matter. The first thing we can do is simply look at the composition of the carbon atoms that make up the organic matter. It turns out that about 99 percent of all the carbon on this planet is so- called carbon 12. It has six protons and six neutrons, but about one percent is another stable form of carbon called carbon 13. It has an extra neutron. You may have heard of carbon 14 which is a rare form of carbon that turns out to be radioactive so it decays over a number of thousands of years and wouldn't be present in a rock this old. The reason we want to know how much carbon 12 and carbon 13 is in this rock is that photosynthetic organisms, when they take carbon dioxide from the environment and fix it into organic matter, preferentially use carbon 12 relative to carbon 13 so organic matter that was formed by photosynthesis will have a ratio of carbon 13 to carbon 12 that is different from that of the environment and measurably different from that of limestones that would form from the same water body. Using a mass spectrometer you can measure the difference and its parts per thousand difference between limestone and organic matter, but that subtle chemical signal is a signature of photosynthesis. So we know that when this rock was deposited, the local ecosystem was driven by photosynthesis.
It turns out there are also organic molecules that are preserved and some of those can actually be traced to specific producing organisms. So, for example, there are some chemicals in here, some molecules that tell us that cyanobacteria were important parts of the photosynthetic biota in this ocean. But interestingly, there are other very specific molecules that tell us that there were other photosynthetic bacteria present in large abundances in the water column beneath which this rock accumulated. Those are so-called green and purple photosynthetic bacteria and what's interesting about those organisms is that they don't use water for photosynthesis and they don't produce oxygen. Instead of using water as their source of electrons, they use hydrogen sulfide, that rotten egg smell that you're all familiar with, and they will only conduct photosynthesis where there is light but no oxygen. So this tells us that a billion and a half years ago the surface waters of the ocean had oxygen and supported cyanobacteria but within several tens of meters of the surface we got to an area that was full of sulfide and had a photosynthetic bacteria in it. In general, what this rock is trying to tell us is that a billion and a half years ago the ocean looked a little bit like the black sea does today - oxygen at the top, sulfide at the bottom. And there's a series of other physical chemical measurements you can make on this rock, things having to do with the amount and mineral distribution of iron in the rocks, things having to do with the isotopes of sulfur in these rocks, things having to do with the amount and isotopes of the metal Molybdenum in these rocks and they all give us the same answer. They say that 1500 million years ago, long after oxygen began to accumulate in the atmosphere, long after the evolution of cyanobacteria, the world still had only a small part of the oxygen in that it has today.
Interviewer: Do you have another rock that bridges that time to today?
ANDY: That's a good question. If we go back to this rock deposited 700 million years ago and ask those same questions about environmental chemistry that we asked about the shale that was twice as old, we find that still, 700 million years ago, the world looks pretty alien. It's still a different world. It's still a world with much less oxygen than we have now and we are finding out, literally even as we're having this conversation, data sets are accumulating that tell us from evidence from all over the world that the first rocks that record a world that's like our own or at least similar to our own in terms of having a lot of oxygen were deposited only about 580 million years ago. So we seem to go from an alien world in which you and I could not have survived very long to a world that would be physically and soon thereafter biologically familiar only in the last fifteen percent of our planet's history. Now you can ask an important question and that is "Why?". Why does the world make that transition to a modern state and the honest answer is there are lots of ideas but no definitive evidence so in a sense, and I don't say that to sound defeatist, rather I think it's exciting, what that means is that there's something we don't know about our planet's history and, therefore, something that we can study and make new discoveries. If we understood everything, there would be no point in being a scientist. Rather, scientists get up in the morning because there are important questions that we can't answer and the scientific method is devising very rigorous ways of trying to answer questions about what we don't know.
Interviewer: So let me ask you, what gets you up in the morning? What do you find is the most burning question that right now is being answered? Are there any clues right here in front of you?
ANDY: There are a number of questions that I'm excited about and they're the things that get me up every morning and bring me into the lab to work with my students. Some of them are really part and parcel of the kind of rocks we've been talking about. This question of why do we go from one long-lived state of the earth that's alien to us to a world that's more similar to us? What combination of biological and physical processes govern that history? That's one of the questions that I hope we can help to answer over the next five years.
A second set of questions that get me excited relate to the following relationship. For many years we've had a fossil record that allowed us to chronicle the history of life. In the last ten or fifteen years, we started to have a much improved way of using the chemistry of rocks to understand earth's physical history and what I really want to understand is the relationship of that physical history to our planet's biological history. That is, when are there physical events that influence the subsequent course of evolution? When are there evolutionary events that feed back onto a changed world? And in order to make that connection, we really want to think about fossils not only as objects with shape and size and sort of mechanical function, but we want to think about fossils as carriers of physiological information. I think that's a type of information that's been underappreciated in considerations of our planet's history, but I think it's really the keystone that's going to allow us to have a much deeper understanding of how physical and biological events have interacted to bring us to our present moment.
Interviewer: To summarize, would you say that there's a transformation going on in the field from merely looking at the physical and the phylogenetic fossils to looking at the chemical signatures and linking that with DNA, for example in bio sequencing?
ANDY: Yes, there's a profound transition going on in this field and it's a transition in which once disparate fields are becoming interlocked and integrated. \A generation ago paleontologists, commonly at least, would not have had lunch with geochemists. Molecular biologists would not have had lunch with geochemists and paleontologists. But now we find that we can interpret our fossils in light of molecular information on genealogy that is accumulating from biological labs. We now are starting to understand the molecular basis of the development of animals from eggs to adults and, therefore, that carries the seeds of understanding how evolutionary changes and form can take place. Chemists are telling us a great deal about the details of our planet's environmental history so we old-fashioned paleontologists now are working with molecular biologists, with geneticists, with chemists, and with planetary scientists because if you accept the idea that life is a planetary phenomenon, you're interested in asking questions not only about Earth and its history but about Mars and its history, about Europa and its history and ultimately, by using remotely sensed information from powerful telescopes, perhaps even about a history of life that may have been played out in another solar system.